High seroprevalence for SARS-CoV-2 among household members of essential workers detected using a dried blood spot assay

Abstract

Objective: Serological testing is needed to investigate the extent of transmission of SARS-CoV-2 from front-line essential workers to their household members. However, the requirement for serum/plasma limits serological testing to clinical settings where it is feasible to collect and process venous blood. To address this problem we developed a serological test for SARS-CoV-2 IgG antibodies that requires only a single drop of finger stick capillary whole blood, collected in the home and dried on filter paper (dried blood spot, DBS). We describe assay performance and demonstrate its utility for remote sampling with results from a community-based study.

Methods: An ELISA to the receptor binding domain of the SARS-CoV-2 spike protein was optimized to quantify IgG antibodies in DBS. Samples were self-collected from a community sample of 232 participants enriched with health care workers, including 30 known COVID-19 cases and their household members.

Results: Among 30 individuals sharing a household with a virus-confirmed case of COVID-19, 80% were seropositive. Of 202 community individuals without prior confirmed acute COVID-19 diagnoses, 36% were seropositive. Of documented convalescent COVID-19 cases from the community, 29 of 30 (97%) were seropositive for IgG antibodies to the receptor binding domain.

Conclusion: DBS ELISA provides a minimally-invasive alternative to venous blood collection. Early analysis suggests a high rate of transmission among household members. High rates of seroconversion were also noted following recovery from infection. Serological testing for SARS-CoV-2 IgG antibodies in DBS samples can facilitate seroprevalence assessment in community settings to address epidemiological questions, monitor duration of antibody responses, and assess if antibodies against the spike protein correlate with protection from reinfection.

Fig 1. Dried blood spot (DBS) ELISA for the receptor binding protein domain of the SARS-CoV-2 spike protein.

A) The CR3022 antibody has known reactivity to the receptor binding domain protein. Measurements from DBS samples to which known concentrations of CR3022 antibody were added. B) Increased signal readily detected in DBS from virus-positive cases. C) Near perfect correlation between DBS and serum IgG levels. *p<0.001.

Fig 2. Results from community collected DBS from April—May 2020.

A) The range of IgG seropositivity detected in DBS samples collected from 30 known virally infected cases (median 28 days after viral test, range 16–43 days) and 202 without documented COVID-19 infection. B) Depicts the lower range of DBS detected seropositivity with those OD greater than 0.6 μg/ml considered positive, and less than 0.39 μg/ml considered negative. The range in between was considered low seropositive.

Fig 3. Seroconversion documented in repeat DBS sampling in non-confirmed SARS-CoV-2 and PCR positive participants.

Increased IgG concentrations in 10/28 (35%) seronegative samples, 5 of which became low positive or seropositive (18%) upon resampling (median 14 days; range14-23 days). After 2–3 weeks (median 14 days; range 13–23 days), 15/19 (79%) low seropositive participants had increased IgG concentrations with 11/19 (58%) becoming seropositive. 13 of 13 (100%) of known SARS-CoV-2 viral PCR+ participants remained low seropositive or seropositive upon resampling (range 14–21 days; median 47 days post positive viral swab test). Dotted grey line marks the low positive cut-off value. * p < 0.05.

Fig 4. High seroconversion rates in household members of index COVID-19 cases.

A) Seroprevalence in 202 samples collected from the community, which includes health care workers and first responders, none of which were confirmed SARS-CoV-2 viral positive. B) Of 30 COVID-19 exposed household members, 70% were seropositive and 10% low seropositive. C) Increased seroconversion in household members of known viral PCR positive healthcare workers (53% seropositive and 18% low positive) compared to 100% seronegative in known viral PCR negative healthcare worker households. Household members of healthcare workers with no viral testing had exposure rates more similar to the community acquired rates.